1. Field of the Invention
The present invention generally relates to a fixing device and an image forming apparatus.
2. Description of the Related Art
Image forming apparatuses such as copiers, printers, and facsimiles of recent years typically form an image by performing an image forming process which may be electrophotographic recording, electrostatic recording, or magnetic recording. Thereafter, an unfixed toner image is formed on a recording medium such as a recording medium sheet, printing paper, photosensitive paper, or electrostatic recording paper using an image transfer scheme or a direct image formation scheme. Known types of fixing devices for fixing such an unfixed toner image include fixing devices that perform a fixing process by applying heat and pressure to a toner image formed on recording paper in a nip area between an endless belt and a pressure roller.
Known examples of such a fixing device include belt-type fixing devices. For example, a belt-type fixing device may be configured as illustrated in FIG. 10. Referring to FIG. 10, a fixing belt 103 is disposed between a heating roller 104, which internally includes a heater 105, and a fixing roller 102. A pressure roller 101 is pressed against the fixing roller 102 with the fixing belt 103 to form a fixing nip area. A tension is applied from a tension applying unit 107 to the fixing belt 103. A separation claw 106 that strips off a recording sheet P where a toner image is fixed from the fixing belt 103 is arranged. The temperature of the fixing belt 103 on the side of the heating roller 104 is detected using a thermistor 110.
In recent years, there is a desire for further reduction in warm-up time and first print time. The warm-up time refers to the time lapse until the temperature rises at power-on or a like occasion from a room temperature to a predetermined temperature (reload temperature) at which printing can be performed. The first print time refers to the time lapse between receipt of a print request and when a sheet of recording medium, on which printing is performed after making preparation for the printing, is ejected. Furthermore, as the output speed of image forming apparatuses increases, the number of sheets fed per unit time increases, and larger amounts of heat is required. This results in a problem that shortage of heat (which may be generally referred to as temperature drop) occurs in particular at beginning of continuous printing. However, the conventional belt-type fixing device fails to fully solve these problems.
To alleviate the problems, a fixing device of the following type is disclosed. The fixing device uses a pipe-like metal heat conductor disposed inside an endless belt. The metal heat conductor is heated by an internal heat source, and the entire endless belt is heated by convection in an air layer between the endless belt and the metal heat conductor, radiant heat, heat conduction, and the like. An example of this type of fixing device that uses such a pipe-like metal heat conductor is disclosed in Japanese Laid-open Patent Application No. 2007-334205.
The fixing device disclosed in Japanese Laid-open Patent Application No. 2007-334205 further includes a pressure roller that is to be brought into contact with the metal heat conductor with the endless belt therebetween, thereby forming a nip area. The pressure roller is rotated, causing the endless belt to be moved in the circumferential direction relative to the fixed metal heat conductor by rotation of the pressure roller. This configuration allows heating the entire endless belt included in the fixing device, so that first print time from a heating-standby state is reduced and the problem of heat shortage that can occur when the number of fed sheets per unit time is large is alleviated.
An example of a fixing device including only a nip forming member, a backup member, and a heat source inside a loop of an endless belt is disclosed in Japanese Laid-open Patent Application No. 2007-233011. The nip forming member includes a sliding surface that makes sliding contact with the endless belt. The backup member is positioned on the side opposite from the sliding surface of the nip forming member and supports the nip forming member.
This fixing device includes a pressure roller configured to press the sliding surface of the nip forming member with the endless belt therebetween to form a nip area, and cause the endless belt to be rotated in a predetermined direction by rotation of the pressure roller. This fixing device can advantageously reduce heat capacity of the heat source and therearound. Furthermore, because the nip area is formed at a portion where the endless belt is directly heated by the heat source, the first print time from the heating-standby mode can be reduced.
As disclosed in Japanese Laid-open Patent Application No. 2007-233011, in the fixing device including, inside the loop of the endless belt, the nip forming member including the sliding surface that makes sliding contact with the inner surface of the endless belt and the backup member supporting the nip forming member, the backup member should pass through the belt. In the fixing device, a nip supporting member which is the backup member that receives a load applied thereto from the pressure roller via the nip forming member functions as a beam with fixed ends (hereinafter, “fixed-ends beam”) that supports the load at its both ends.
Meanwhile, for reduction in size and elimination of unnecessary heat capacity of a fixing device, the smaller the volume of the nip supporting member, the more desirable. However, reducing the volume of the nip supporting member can arise the following problem. Assume that, as illustrated in FIG. 11A, a nip forming member 122 inside a loop of an endless belt 121 is supported by a nip supporting member 123, which is supported at both ends by side plates 125 on an apparatus body. When a pressing force Pr is applied from a pressure roller 124 to the nip supporting member 123 in this state, a center portion of the nip supporting member 123 is warped in a direction (downward in FIG. 11A) away from a nip area N. As the volume of the nip supporting member decreases, rigidity of the nip supporting member 123 decreases, and warping of the nip supporting member 123 increases. As a result, as indicated by the dashed line in FIG. 11B, a width n of the nip area N of the nip forming member 122 decreases at its longitudinal center portion, making a nip width that is accurately uniform in the longitudinal direction of the nip forming member unobtainable.
If a uniform nip width cannot be obtained because the nip width n of the nip forming member 122 varies in the longitudinal direction of the nip forming member 122, stable fixing quality becomes unachievable.
To alleviate this problem, a configuration that compensates for warping of the nip forming member 122 even if the nip forming member 122 is warped by the pressing force Pr applied from the pressure member may be employed. Put another way, a fixing device is desirably configured to obtain a nip width that is accurately uniform in the longitudinal direction of a nip forming member even if the nip forming member that faces a pressure member to form a fixing nip area therebetween is warped by a load applied from the pressure member.
Therefore, it is desirable to provide a fixing device in which a nip forming member can have a nip width that is accurately uniform in the longitudinal direction, thereby providing stable fixing quality, and an image forming apparatus including the fixing device.